The present invention relates to a vertical low pressure CVD (Chemical Vapor Deposition) apparatus for effecting vapor phase epitaxy on semiconductor substrates or wafers.
A vertical low pressure CVD, or sometimes abbreviated as LPCVD, apparatus is disclosed in, for example, Japanese Laid-Open Publication No. 1-73930. This kind of conventional CVD apparatus forms films on wafers by using monosilane gas and oxygen gas as reactive gas. In the conventional CVD apparatus, an annular nozzle formed with holes for jetting monosilane gas is disposed remote from the oxygen inlet port, so that the monosilane gas is prevented from reacting with the oxygen gas, which is highly oxidizing, at an early stage. Moreover, the oxygen gas inlet port is formed through only one portion of a flange which is connected to the bottom of an outer tube. When the pressure inside the CVD apparatus is below 1 Torr, the molecules of gas flow in a transitional range between a viscous flow range and a molecular flow range, as is well known in the art. Hence, the diffusion of oxygen molecules jetted from the oxygen gas port and flowing toward the annular nozzle is caused to change by the circumference of the annular nozzle. As a result, the mixture of monosilane gas and oxygen and, therefore, the reaction of the two kinds of gas becomes irregular. Specifically, in a region where the monosilane concentration is high, the two kinds of gas react excessively and roughen the surfaces of the wafers. Conversely, in a region where the monosilane concentration is low, the reaction is short and reduces the thickness of the resulting films.